Safety Factor for Drilled Shaft Foundations Subjected to Wind-Induced Torsion-Article

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The study develops reliability-based factors of safety for drilled-shaft foundations subjected to wind-induced torsion, addressing the lack of guidance in AASHTO specifications. Current agency practices use widely varying safety factors that generally ignore site-specific soil uncertainty, potentially causing either unsafe designs or unnecessary construction costs.<br /><br />The authors evaluate torsional resistance using skin-friction methods: the α method for cohesive soils and the β method for cohesionless soils. They quantify uncertainty in soil properties, construction dimensions, and analytical predictions using published statistics, experimental test data, and Monte Carlo simulations. Wind effects are also modeled probabilistically. Because torsional failure of traffic-sign and signal foundations is generally a serviceability concern rather than a catastrophic, life-threatening event, the authors recommend a target reliability index of 1.5–2.0.<br /><br />Recommended factors of safety vary with the target reliability and the soil-strength test used. For a target reliability index of 1.5, the suggested factors are 1.60 for the α method using SPT data, 1.40 using CPT, 1.20 using vane shear tests, and 0.95 for the β method (with a factor above 1.0 generally advised). For a target index of 2.0, the corresponding values are 2.20, 1.85, 1.50, and 1.15. The authors provide a procedure for selecting factors for layered soils and recommend a piecewise approach, which performs more consistently than a single or weighted-average factor.<br /><br />The procedure can be adapted to local site data and other reliability targets. However, the authors emphasize that field test data on torsional resistance remain limited and further experimental research is needed.

Keywords

drilled-shaft foundations

wind-induced torsion

reliability-based design

factor of safety

AASHTO specifications

soil uncertainty

Monte Carlo simulation

skin-friction resistance

alpha method

beta method